During the drilling of a wellbore, various fluids are typically used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore, and then may subsequently flow upward through wellbore to the surface. During this circulation, the wellbore fluid may act to remove drill cuttings from the bottom of the hole to the surface, to suspend cuttings and weighting material when circulation is interrupted, to control subsurface pressures, to maintain the integrity of the wellbore until the well section is cased and cemented, to isolate the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, to cool and lubricate the drill string and bit, and/or to maximize penetration rate.
In most rotary drilling procedures the wellbore fluid takes the form of a “mud,” i.e., a liquid having solids suspended therein. The solids function to impart desired rheological properties to the wellbore fluid and also to increase the density thereof in order to provide a suitable hydrostatic pressure at the bottom of the well. The wellbore mud may be a water-based, an oil-based, or a synthetic-based mud.
Many types of fluids have been used in well bores particularly in connection with the drilling of oil and gas wells. The selection of an oil-based/synthetic-based wellbore fluid involves considering the characteristics of such fluids in a particular application. The primary benefits of selecting an oil-based/synthetic-based wellbore fluid include: superior hole stability, especially in shale formations; formation of a thinner filter cake than the filter cake achieved with a water based mud; excellent lubrication of the drilling string and downhole tools; penetration of salt beds without sloughing or enlargement of the hole as well as other benefits that should be known to one of skill in the art. A beneficial property of oil-based/synthetic-based wellbore fluids is their excellent lubrication qualities. These lubrication properties permit the drilling of wells having a significant vertical deviation, as is typical of off-shore or deep water drilling operations or when a horizontal well is desired. In such highly deviated holes, torque and drag on the drill string are a significant problem because the drill pipe lies against the low side of the hole, and the risk of pipe sticking is high when water based wellbore fluids are used. In contrast, oil-based wellbore fluids provide a thin, slick filter cake which helps to prevent pipe sticking and thus the use of the oil-based wellbore fluids can be justified.
Oil-based and synthetic-based wellbore fluids are generally invert emulsions. The components of the invert emulsion fluids include an oleaginous liquid such as hydrocarbon, vegetable, or synthetic oils which serves as a continuous phase, a non-oleaginous liquid such as water or brine solution which serves as a discontinuous phase, and an emulsifying agent. The oil/water ratio of invert emulsion fluids is traditionally within the range of 60/40 to 85/15. As used herein, emulsifying agent and surfactants are used interchangeably. The emulsifying agent serves to lower the interfacial tension of the liquids so that the non-oleaginous liquid may form a stable dispersion of fine droplets in the oleaginous liquid. Additionally, such invert emulsion wellbore fluids generally contain one or more weighting agents, surfactants, viscosiflers, fluid loss control agents or bridging agents.
The drawback to use of invert emulsion fluids, particularly oil-based or synthetic-based wellbore fluids, is their value (due to the oil content) and some environmental concerns associated with the disposal of oils. Therefore, wellbore fluids are typically recycled or reused.